The present invention relates to nanoparticulate, redispersible zinc oxide gels, to processes for their preparation and to their use.
Zinc oxide is used for diverse purposes, such as, for example, as a white pigment, as a catalyst, as a constituent of antibacterial skin-protection ointment and as an activator for rubber vulcanization. Sunscreens and wood varnishes contain finely divided zinc oxide as UV-absorbing pigment.
The term xe2x80x9cnanoparticlexe2x80x9d is generally used to refer to particles having a diameter of less an about 100 nm.
Zinc oxide nanoparticles having particle sizes below about 30 nm are potentially suitable for use as UV absorbers in transparent organic-inorganic hybrid materials, plastics, paints and coatings. In addition, use for the protection of UV-sensitive organic pigments is also possible.
Particles, particle aggregates or particle agglomerates of zinc oxide which are greater than about 30 mn lead to scattered light effects and thus to an undesired decrease in transparency in the visible light region. For this reason, redispersibility, i.e. the convertibility of the prepared zinc oxide nanoparticles into a colloid-disperse state, is an important prerequisite for the abovementioned applications.
Zinc oxide nanoparticles having particle sizes below about 5 nm exhibit, because of the size quantization effect, a blue shift in the absorption edge (L. Brus, J. Phys. Chen. (1986), 90, 2555-2560) and are therefore less suitable for use as UV absorbs in the UV-A region.
The preparation of zinc oxide by dry and wet processes is known. The classical method of burning zinc, the dry process (e.g. Gmelin vol. 32, 8th edition, supplementary volume, p. 772 ff), generates aggregated particles having a broad size distribution. Although in principle it is possible to prepare stable dispersions by grinding procedures with the help of surface-active agents, because the shear forces which can be achieved are too low, it is not possible to obtain dispersions having average particle sizes below about 30 nm from such powders.
Particularly finely divided zinc oxide is prepared predominantly by wet chemical methods by precipitation processes. Precipitation in aqueous solution generally gives hydroxide- and/or carbonate-containing materials which have to be thermally converted to zinc oxide. The thermal post-treatment has a negative effect on the finely divided nature since the particles are subjected during this treatment to sintering processes which lead to the formation of xcexcm-sized aggregates which can be broken down only incompletely to the primary particles by grinding.
JP-A-04 164 814 describes a process which leads, as a result of precipitation in aqueous medium at elevated temperature even without thermal post-treatment, to finely divided ZnO. The average particle size is given as 20-50 nm, with no information on the degree of agglomeration. These particles are relatively large. Even if agglomeration is minimal, this leads to scattering effects which are undesired in transparent applications.
JP-A-07 232 919 describes the preparation of ZnO particles which are 5-10,000 nm in size from zinc compounds by reaction with organic acids and other organic compounds such as alcohols at elevated temperature. The hydrolysis is carried out here such that the by-products which form (esters of the acids used) can be distilled off. The process permits the preparation of ZnO powders which are redispersible as a result of surface modification which has been carried out beforehand. However, on the basis of the disclosure of this application it is not possible to prepare particles having an average diameter of  less than 15 nm. Accordingly, in the examples given in the application, the smallest average primary particle diameter is given as 15 nm.
EP 0 893 409 A1 describes the preparation of zinc oxide nanoparticles as in JP-A-07 232 919, except that during the precipitation of the ZnO, another metal oxide, from the groups of the Periodic Table of the Elements named xe2x80x9cIIIBxe2x80x9d and xe2x80x9cIVBxe2x80x9d in the application, aluminum and indium being given in particular by name, is coprecipitated.
Furthermore, attempts have been made to obtain ZnO directly by hydrolysis of zinc salts in alcohols (Henglein et al., J. Phys. Chem. 1988, 92, 482-487). Using NaOH as base it was not possible to obtain stable and concentrated sols (CZnO greater than  greater than 10xe2x88x923 mol/l).
The hydrolysis of zinc acetate with LiOH or tetramethylammonium hydroxide (Spanhel et al., JACS 1991, 113, No.8, 2826-2833) in alcoholic solution gave concentrated sols which, in addition to zinc oxide nanoparticles, also comprised lithium acetate or tetramethylammonium acetate, respectively, in stoichiometric amount. For this reason, firstly, cost-effective preparation is not possible since LiOH and tetramethylammonium hydroxide are relatively expensive, and, secondly, further use of the sols is severely limited since the by-products of the precipitation are not separated off. By concentrating these sols it was possible to obtain ZnO-containing gels which likewise still contained the by-products of the ZnO preparation in stoichiometric amount and were thus severely limited with regard to their further possible uses.
Other processes for the preparation of nanosize zinc oxide, such as that described in U.S. Pat. No. 5,391,354, which starts from zinc alkoxides, use expensive starting materials and are therefore uneconomical.
The object of the present invention was therefore to provide a nanosize zinc oxide which combines strong UV-absorption even in the UV-A region with excellent dispersion properties for minimal scattering To this end, it was necessary to find a simple process which is suitable for preparing nanosize zinc oxide having an average primary particle diameter between 5 and 15 nm from commercially available, cost-effective starting materials on an industrial scale at low cost in a manner which permits the zinc oxide, following preparation, to be separated off from the by-products without the particles undergoing irreversible aggregation and, following redispersion, to be prepared for further use in the form of sols without laborious grinding.
Starting from the method for the hydrolysis of zinc acetate in alcoholic media described by Henglein et al., it has now surprisingly been found that by hydrolysing zinc compounds with bases in alcohol or alcohol/water mixtures, followed by removal of the supernatant solution charged with the by-products of the precipitation, it is possible to obtain zinc oxide gels comprising zinc oxide nanoparticles having average primary particle diameters of xe2x89xa615 nm which, simply by adding suitable solvents or solvent combinationsxe2x80x94optionally together with suitable surface modifierxe2x80x94sand stirring, and dispensing with grinding stages or other laborious mechanical comminution processes, can be redispersed completely to give largely primary-particle-disperse zinc oxide sols, without a significant loss in quality with regard to the monodispersity and size of the particles.
The invention therefore relates to zinc oxide gels comprising nanosize zinc oxide particles having an average primary particle diameter of xe2x89xa615 nm, water and alcohol, which are redispersible in at least one organic solvent and/or water, optionally with the addition of surface-modifying compounds, to give sols.
For the purposes of the invention, xe2x80x9caverage primary particle diameterxe2x80x9d means the average circle-equivalent primary particle diameter of the zinc oxide particles which can be determined in a transmission electron micrograph.
The invention also relates to a process for the preparation of zinc oxide gels by basic hydrolysis of at least one zinc compound in alcohol or an alcohol/water mixture, characterized in that the precipitate which initially forms during hydrolysis is left to mature until the zinc oxide has completely flocculated, then this precipitate is thickened to a gel and separated off from the supernatant phase.